1. The Technical Field of the Invention
The present invention concerns a method of introducing shear forces into a concrete body, in particular, through the side face of a concrete slab, in which the force is absorbed by an axially extended shear force rod, e.g. a shear force mandrel or a sleeve around the shear force mandrel, is diverted from the shear force rod by a shear force strap connected to the shear force rod at at least one point close to the surface of the concrete and one point distant from the surface of the concrete and is transmitted to the concrete.
The invention also concerns an element for introducing shear forces into a concrete body, in particular, through the side face of a concrete slab, with an axial shear force rod, e.g. a shear force mandrel or a sleeve around the shear force mandrel, and with a shear force strap fixed to the shear force rod at at least one point close to the surface of the concrete and one point distant from the surface of the concrete, for transmitting shear force to the concrete, and it also concerns a concrete body with a shear force element according to the invention.
2. Brief Description of Art
The most diverse forms of shear force absorbing pins are known, in which the cross section of the pin is increased in the stressed zones in order to reduce the local compressive forces occurring at the transition between pin and concrete. However, the load capacity of the concrete slab is reduced by the proximity of these pins to the surface of the concrete on the compression side.
An xe2x80x9cInvestigation into shear pins of reinforcing steel embedded in concretexe2x80x9d published in 1983 in Vol. 346 of the xe2x80x9cDeutscher Ausschuss fxc3xcr Stahlbauxe2x80x9d examined ten different reinforcements by which the shear forces introduced into the side face of a concrete body could be distributed in order to increase the load capacity of a concrete slab in the zone of the shear forces introduced. It emerged from this that only two of the test arrangements examined were fully effective. These were two-shear, rearwardly anchored loops in direct centric contact with the shear pin (p. 144, right column, paragraph 2). These loops were made of round steel bars at least 10 mm thick.
Because unified solutions are often more economic than individual ones on construction sites, prefabricated elements have been developed which can be incorporated in the shuttering as ready-made products. In the specialized trade two elements for introducing shear forces are known in particular, and are described below with reference to FIGS. 1 to 3.
The shear force strap in FIGS. 1 and 2 is shown cast into a concrete slab 10 and has a steel plate 13 placed in the side face of the concrete slab 11 and two straps 15, 15xe2x80x2 welded to this steel plate. Plate 13 has a central opening 17 in which a mandrel 19 is inserted. The two straps 15, 15xe2x80x2 are set parallel to each other and away from the concrete surface of plate 13 and, at a distance from the mandrel axis 21, are bent in a direction parallel to the mandrel axis 21. The end portions 23 of the reinforcing steel forming the straps 15, 15xe2x80x2 are bent through about 165 to 170 degrees at a relatively large distance from the side face of the slab 11 and return to meet the end of mandrel 19 about halfway back to the side face of the slab. The straps 15, 15xe2x80x2 are formed symmetrically to axis 21, so that shear forces in two opposite directions can be transmitted to the concrete slab by the same strap. The four ends 23 of the straps are linked to each other and to the mandrel 19.
A part of the force (arrow 25) introduced into this strap via the shear force mandrel is transmitted as tensile force to the upper part of strap 15. Because of the eccentric introduction of the force into the mandrel, said mandrel has a tendency to twist away, so that it transfers compression forces to the ends 23 of the upper part of straps 15, 15xe2x80x2. Bending forces act, in particular, on the horizontal part of the strap. In addition, a part of the force introduced is transmitted to strap 15 as compression force in the lower part of strap 15. This compression force is transmitted to the concrete slab 10 in the vicinity of the underside 27 of the slab.
The strap 15 shown in FIGS. 1 and 2 is a two-shear loop anchored rearwards with direct centric contact to the mandrel, and differs from the arrangements which have proven effective in the study mentioned only in that the rearwardly anchored reinforcing rods 15, 15xe2x80x2 are in centric contact with the mandrel by means of a plate 13 and in that the ends 17 are bent round and fixed to the mandrel.
The device 31 shown in FIG. 3 acts in a similar way to the device in FIGS. 1 and 2. Tensile and compressive forces are transmitted by the shear force mandrel 32 via the rigid plate 33 on the side face of a concrete slab to the two arms 35 of the flat steel strap 37 formed symmetrically on either side of the mandrel 32, which arms are welded to plate 33. Bending forces which are difficult to calculate occur in the strap arms 35, which extend into the concrete in a convergent direction from plate 33 and are fixed to the shear force mandrel 32 at a distance from plate 33. Here, too, the compressive forces are transmitted to the concrete lying in the direction of the pressure close to the surface of the concrete.
A disadvantageous feature of these prefabricated elements is that compressive forces arise close to the surface of the concrete on the compression side. There is therefore a danger that fragments of concrete will split away. A further disadvantage is that the resulting flow of forces in the concrete remains unclear. The forces arising can hardly be calculated, since the calculations cannot be based on any simple model.
It is therefore the objective of the invention to create a method and a device for introducing shear forces into a concrete body whereby the introduction into the concrete of compressive forces close to the surfaces of the concrete and directed towards these surfaces is avoided. Furthermore, the device shall be capable of being formed symmetrically, so that incorrect insertion of the device on the construction site is made impossible. In addition, the forces are to be calculable according to a simple model.
This objective is achieved according to the invention in that, in a method of the type mentioned at the outset, the strap is so formed around a concrete core forming part of the concrete body that substantially only tensile forces can arise in the strap. The strap is advantageously formed in an arc around the concrete core. Thereby the compressive forces in the concrete occur inside the arc section. As the arc section is arranged on the tension side, and if the shear force rod is arranged at approximately the center of the thickness of the slab, a larger part of the slab""s thickness certainly lies on the compression side than on the tension side of the strap.
The arc, preferably of flat material, is advantageously formed symmetrically around the concrete core. The symmetry permits simpler calculation of the forces arising, as the components aligned perpendicularly to the axis of symmetry cancel each other out. The sum of the force vectors therefore forms a vector on the axis of symmetry. The loading on the concrete is especially consistent when a planar arc section is used, as only compressive forces arise in the concrete, similarly to those in a vault. A planar arc section has the advantage of smaller local compressive forces and a three-dimensional play of forces, increasing the local load capacity of the concrete.
Advantageously, a first strap or strap section connected to the shear force rod is formed around a concrete core close to the surface of the concrete, and a second strap or strap section connected to the shear force rod is formed around a concrete core distant from the surface of the concrete. In this way the moment arising from the eccentric loading of the shear force rod can be absorbed.
Advantageously the tensions arising in the concrete core are distributed by means of an additional reinforcement in the concrete. This is especially indicated in a corner of a concrete slab, as the forces arising in the concrete in that area perpendicularly to the side face of the concrete slab are directed outside the face set at an angle to it beyond the corner of the slab. These outwardly directed compressive forces must be absorbed by a reinforcement. But the compressive forces arising at a greater distance from a corner are also advantageously distributed by reinforcements in the concrete slab. Normally such a reinforcement comprises standard slab edge strapping with U-shaped straps, the legs of which are arranged parallel to the plane of the slab and perpendicularly to the face of the slab. At a corner the straps are arranged at an angle to each other corresponding to the angle of the corner and are interlocked. The legs of the straps can also be connected by steel bars arranged parallel to the face of the slab. Advantageously, however, fiber reinforcement is used, as this also increases the compressive strength of the concrete.
In the case of an element for introducing shear forces into a concrete body, especially through the side face of a concrete slab, with an axial shear force rod, e.g. a shear force mandrel or a sleeve around the shear force mandrel, and with a shear force strap attached to the shear force rod at at least one point close to the surface of the concrete and one point distant from the surface of the concrete, the strap according to the invention is a loop element for transmitting shear force to the concrete, the sections of which element attached to the shear force rod are linked by an arc section at a distance from the shear force rod.
The strap is outstandingly suited to translating tensile forces in the arc element into compressive forces in the core contained inside the arc. Advantageously, the loop element is flexible enough under load from the forces arising that is absorbs substantially only tensile forces. It can comprise, for example, a chain, a steel cable or steel mesh, sheet metal, or a glass fiber or carbon fiber structure or suchlike element. A fixed connection with high friction between the loop element and concrete is not desirable, as the loop, because of its form and loading, is intended to transmit only compressive forces to the concrete core which it encloses.
The loop element preferably has a symmetrical arc section, e.g. an arc section with, for example, a circular, elliptical or parabolic arc. The forces in such a symmetrical or geometrically defined arc section, and the forces induced in the concrete by this arc section, can be calculated with simple models.
The loop element advantageously has a curved planar element in the arc section zone, so that the local forces are as small as possible.
The loop element is preferably formed of a strip which is relatively wide in relation to its thickness. Such a strip can absorb practically no compressive forces in the direction of the strip, as it is too thin. Its total cross section, however, permits appropriate tensile forces in the strip, so that high pressures can be transmitted to a concrete core around which the strip is formed. Because of the flexibility of the strip it can be assumed when making calculations that the strip is formed around a virtual roller, so that no unilateral tension on the strip is possible. For this reason the distribution of pressure in the concrete core inside the loop is very simple.
The arc section is advantageously curved substantially around an axis parallel to the surface of the concrete, and at least one of the sections of the loop element connected to the shear force rod is so bent or twisted that the bent or twisted part touches the shear force rod along a line or plane parallel to the axis of the shear force rod. Because of the parallelism of the arc axis to the surface of the concrete, the forces are distributed parallelly to the face of the concrete. For this reason the compressive forces are not directed towards the surface of the concrete, which is practically incapable of absorbing such compressive forces. Because of the linear or planar contact between the shear force rod and the loop element a very good possibility of attachment is provided.
Advantageously, the two sections of a loop element attached to the shear force rod run practically parallel. As a result, especially if the two sections are arranged at a short distance from each other, an even distribution of forces in the loop can be assumed. Alternatively, the section close to the surface of the concrete is aligned parallel to the surface of the concrete, i.e. to the side face of the concrete slab, and the section of the arc section distant from the surface of the concrete is aligned towards the interior of the concrete and towards the shear force rod and away from the side face. In this way the resulting compressive load on the concrete core surrounded by the loop is a vector directed towards the interior of the concrete.
Advantageously, holes or eyes are arranged in the section of the loop element close to the surface of the concrete and connected to the shear force rod, so that the shear force element can be attached to concrete shuttering with nails through the holes. For this purpose the section of the strap close to the surface of the concrete advantageously projects in one place beyond the corresponding point of the section distant from the concrete surface, and holes are provided in the strap at this projecting point, so that the strap can be attached to shuttering through the holes.
The loop element is advantageously formed symmetrically in relation to the axis of the shear force rod, so that shear forces in two opposite directions can be conducted into the concrete. In addition, this enables the element to be correctly positioned on the construction site with greater security.
The shear force rod advantageously has a zone extending into the concrete which is about twice as long as the distance between the two fixing points of the loop element to the shear force rod, or, if applicable, between the two fixing points closer to the surface of the concrete, so that practically the same stress can be assumed at both attachment points of the loop element.
Advantageously, an arc section is arranged rearwards of the arc section close to the surface of the concrete, which first arc section is aligned in the opposite direction. This inner arc section absorbs transverse forces in the same way as the outer section, but acting in the opposite direction. This prevents rotation of the shear force rod under eccentric loading of the rod.
Advantageously, the diameter of the arc section lying parallel to the axis of the shear force rod is smaller than the distance of the furthest point of the arc section from the shear force rod. In this way the largest compressive forces are applied on the tension side of the shear force rod and thus are applied at a relatively large distance from the surface of the concrete on the compression side.
A concrete body with a shear force element according to the invention is advantageously reinforced by a fiber reinforcement in the zone around the shear force elements. The fiber reinforcement can increase both the tensile and the compressive strength of the concrete.
The concrete body is advantageously a prefabricated element and reinforcements project from the element by means of which the element can be combined with a concrete body cast on site. Through the use of a prefabricated element the separate treatment of the edge zone of a concrete body when casting the concrete body can be avoided. Therefore, monitoring on site of the feeding of the fibers into the concrete into which the shear force elements are cast is not required. Prefabrication permits efficient manufacture of the edge elements under conditions which allow their quality to be guaranteed.